Akonadi: avoid 100% CPU on reading corrupt files.

Summary:
I wonder if the file format changed? I ended up with entityCnt being a very large number,
although the .dat file was very small. In 4 different resources, after migrating from
Leap 42.2 to akonadi master.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: mlaurent, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D3672